audiogroundisolation
In every audio mixer schematic that I have found, the signal circuit is described extensively. Here are a few examples:
However, these diagrams do not show any considerations for the ground connections or isolation for the input's grounds. Is this required? Can all of the grounds for all of the inputs be connected together without creating any signal contamination, or should there be some resistance (or other element) added?
It's been quite some time, so I'll go ahead and answer my own question in case any other nutjob with stupid questions comes through here:
YES, YOU NEED TO TIE ALL GROUNDS TOGETHER!
I breadboarded the prototype and it all worked fantastic, without any hum whatsoever. Star grounding is a good idea to ensure you won't have any ground loops.
Also, I added an optocoupler into the design, to separate digital and analog sections. Will update answer after completing prototype.
You need a simple mixer circuit based on the summing amplifier.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Stereo mixer basic schema.
As Brian Drummand says, you need a virtual earth mixer. Here's the basic schematic showing how to handle the mono input.
This diagram does not show power-supply and capacitors. You'll find details in a web search.
Best Answer
No. In fact, the mixer usually functions as the central grounding tie point for the entire system.
For the inputs in particular, which are mostly connected to microphones that have no other ground reference, this is particularly important.
Only in special cases, such as taking an input from a separately-powered instrument amplifier, would you consider using a transformer or other form of isolation (i.e., a "direct box") to separate the grounds.